Over time, the repeated normal use of electrical equipment causes electrical connections to become loose and weak. The increased electrical resistance of such connections can result in heating which presents a fire hazard. Insurance and utility companies have recommended guidelines that establish temperatures which indicate that corrective action should be taken. In the past, inspection of electrical equipment for such faults was unsafe, costly and/or inaccurate. Infrared inspection has been used as a fast, easy and very effective way of detecting electrical problems by the heat generated. Infrared inspection is safe as no contact is made with the electrical equipment. It is cost effective and accurate as the equipment is not shut down. Infrared inspection can also be used to detect fire from sources other than electrical equipment.
One such infrared detector is the Microscanner E manufactured by EXERGEN Corp. of Natick, MA. That detector comprises a multicolor full bar graph display which responds to a radiation sensor and provides a temperature signal of a subject above a reference temperature. The bar graph display is arranged into three segments of about zero degrees centigrade to 9 degrees centigrade, about 10 degrees to 19 degrees centigrade and about 20 degrees centigrade and greater. Each segment is of a different color. It is preferred that the segments are colored green, yellow and red respectively, indicating the recommended guidelines of insurance and utility companies for the indicated temperature rise above ambient temperature. The green segment indicates an acceptable temperature rise. The yellow segment indicates a potential problem and that reinvestigation is needed. The red segment indicates an immediate problem. The segments are illuminated from zero degrees to the indicated temperature to give a full bar indication of the amount of radiation sensed by the radiation sensor. The red segment flashes on and off when a temperature rise above 100 degrees centigrade is detected.
The display includes elements which are driven in a piecewise linear fashion. The elements are grouped together and each group is associated with a display driver. The display drivers respond to a display input indicative of the amount of radiation sensed by the radiation sensor and incrementally select display elements. Each driver responds to a single incremental change in the display input for different selections of display elements, but different drivers respond to different incremental changes for different selections of display elements. Thus, the display drivers provide a piecewise linear approximation of temperature which is a nonlinear function of the sensed heat flux.
The display provides a wide range of temperatures by each group of elements being divided into different temperature increments. Specifically, the 20 degrees centigrade and above segment of the bar graph display is divided into larger temperature increments than the 0 degrees to 9 degrees centigrade and the 10 degrees to 19 degrees centigrade segments.
The detector establishes the reference temperature by an autozero circuit. The autozero circuit initially establishes a high reference signal which results in a display signal above the zero level of the bar graph. The high reference signal is reduced until the display signal reaches the zero level of the bar graph at which point the reference signal is held. During a subsequent scan of the target, the autozero circuit sums the reference signal with a radiation signal indicative of the amount of sensed radiation. This sum provides the display signal.